In recent years digital modulation systems were developed in the field of audio, video and data transmission. In digital worldwide broadcasting a common standard is the orthogonal frequency division multiplexing (OFDM) modulation system. The OFDM modulation systems are robust to multi-path interference. In the OFDM modulation system, a transmitted signal includes a large number of subcarriers, reflective of the OFDM mode. For example, in the DVB-T 2k OFDM mode the transmitted signal includes 1,705 subcarriers, while in the DVB-T 8k OFDM mode there are 6,817 subcarriers. The subcarriers are orthogonal to one another, and each is modulated with the digital data to be transmitted. Mathematically, orthogonality is a property of a set of functions such that the integral of the product of any two members of the set, taken over the appropriate interval, is zero. For example, trigonometric functions appearing in Fourier expansions are orthogonal functions.
Reference is now made to FIG. 1A where an illustration of an OFDM frame structure is shown. The transmitted signal in an OFDM modulation system is organized in frames. Each frame includes a constant number of symbols. Each symbol is constructed from multiple subcarriers. In FIG. 1B an illustration of a single symbol structure in the time domain is shown. A symbol includes two parts: a useful part (110) and guard interval part (120). The duration time (Ts) of the useful part is larger than that of the guard interval duration (Δ). A guard interval does not form an integral part of the data to be sent by the transmitter but is rather “artificially” prepended to the useful part during preparation of data for transmission. The guard is therefore redundant and is, as a rule, identified and removed at the receiver end. The subcarriers in a symbol are placed in certain constant frequency intervals, and are represented as a complex number each having a different frequency. A symbol is represented as the sum of subcarriers from subcarrier 0 to subcarrier N−1.
Each symbol includes (in its useful and guard parts) data subcarriers and pilot subcarriers (shown, schematically, in FIG. 1B). As is known per se, pilots subcarriers are used for frame synchronization, time synchronization, frequency synchronization, and channel estimation. Additionally, pilots may be used for phase errors correction, as described, e.g. in “European Telecommunications Standards Institute (ETSI)—ETS 300 7444, “Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for digital Terrestrial television (DVB-T)”, March 1997. Each symbol contains three different types of pilots: scattered, continual, and transmission parameters signaling (TPS). The pilots and the data sub-carriers are shown in the frequency domain in FIG. 1B as vertical arrow symbols (e.g. 130 and 140). Scattered pilots are spread equally in the symbol and between sets of symbols in a frame. For example, the scattered pilots' positions may be defined by using the equation k=(3*(l mod 4)+12*p) where l is the symbol number, k is the frequency slot position, and p is an arbitrary integer number. According to the equation, scattered pilots are inserted at intervals of twelve subcarriers. Scattered pilots of a symbol are shifted, e.g. by three positions relative to the immediately preceding symbol. According to the equation there are four sets of scattered pilots. Symbols further include a constant number of continual pilots, which are placed at fixed locations in each symbol. The number of continual pilots and their positions depends on the OFDM mode. The TPS pilots are used for the purpose of signaling parameters related to the transmission scheme, which is the channel coding and modulation. The TPS pilots may convey the following information: OFDM transmission mode, the value of the guard interval, frame number, and so on. Reference is now made to FIG. 2 where an exemplary block diagram of a hitherto known OFDM receiver 200 is shown (e.g., “Speth M., Fechtel S., Fock G. and Meyr H., “Optimum Receiver Design for OFDM-based Broadband Transmission—Part 2: A Case Study”, IEEE, Vol. 49, No. 4, April 2001.”). Receiver 200 is composed of the following units: analog front-end (AFE) 210, demodulator 220, guard removal unit 230, pre-FFT estimation unit 240, FFT unit 250, equalizer 260, and post-FFT estimation unit 270.
In operation, receiver 200 receives a modulated signal in the time domain, and transforms it to a demodulated signal in the frequency domain. The transformation is done using the fast Fourier transform (FFT) algorithm, and by means of FFT unit 250. AFE 210 converts the received signal from analog form to digital form. Additionally AFE 210 converts the received signal frequency from radio frequency (RF) to intermediate frequency (IF). Demodulator 220 converts the received signal frequency from IF frequency to base-band frequency. Additionally, Demodulator 220 corrects the sampling clock offset and the carrier frequency offset according to the feedback from the estimation units. Guard removal unit 230, is used for removal of the guard-interval portion (e.g. 120) from the received symbol, as well as for timing synchronization. Timing synchronization is the task of locating the beginning of a received OFDM symbol. The timing synchronization ensures a synchronized operation of the transmitter (not shown) and the receiver. In the case of, e.g., digital television system, proper synchronization will facilitate display of qualitative images at the subscriber's screen on the basis of processing the useful parts of the symbols (110 in FIG. 1B). In contrast, when failing to properly synchronize the transmitter and the receiver (i.e. the receiver fails to identify the beginning of the useful data part), the inevitable result is that certain part of the (redundant) data guard (120) is processed (as it is erroneously identified by the receiver as belonging to the useful part of the symbol) and, obviously, degraded quality of images are obtained and displayed. In order to accomplish timing synchronization, a timing error is calculated, i.e. the time difference between transmission and receipt of the useful part for a given symbol.
Post-FFT estimation unit 270 estimates the errors of timing synchronization, frequency synchronization, and sampling-clock synchronization, and performs channel estimation. Additionally, post-FFT estimation unit 270 provides a feedback to the relevant units with a correction signal. AFE 210 is provided with the sampling-clock error, demodulator 220 is provided with the frequency synchronization error, and guard removal 230 is provided with timing synchronization error. In order to provide a fast and reliable acquisition of information sent using OFDM, pre-FFT estimations are required. However, the pre-FTT estimations provide only coarse estimation for timing and frequency synchronizations. The coarse pre-FFT estimations are performed by pre-FFT estimation unit 240. One of receiver 200 tasks is to recognize ghost symbols. A ghost symbol is the reflection of the transmitted signal on various obstacles found in a broadcast environment.
Known implementations of pre-FFT synchronization algorithms provide only coarse estimation of fine synchronization. This has significant shortcomings, e.g. an inaccurate estimation of, say, the timing error and deferring the accurate determination thereof (and the subsequent timing synchronization) to the later (and slower) post FFT unit(s).
Additionally, hitherto known pre-FFT algorithms are not capable of detecting a common phase error (CPE) in the received signal at time domain.